The present invention relates to polymeric electret materials and, more particularly, the present invention relates to polymeric electret filtration materials.
Nonwoven fabrics, fibrillated films, and other materials comprising polymeric fibers or fibrils have been utilized in a variety of filtration and/or air-masking type applications. For example, U.S. Pat. No. 5,709,735 to Midkiff et al. discloses the use of a nonwoven web for HVAC (heating, ventilating and air-conditioning) and other air filtration media. PCT Application No. U.S.94/12699 (Publication No. WO95/13856) discloses high-loft multicomponent fiber webs suitable for use in a variety of air filtration applications. Additionally, U.S. Pat. No. 5,855,784 to Pike et al. discloses a variety of conjugate fiber nonwoven webs suitable for use as air and/or liquid filtration media. Further, multilayer laminates have likewise been used in a variety of filtration and/or filtration-like applications, see, for example, U.S. Pat. No. 5,721,180 to Pike et al. and U.S. Pat. No. 4,041,203 to Brock et al.
Filtration materials desirably exhibit the highest filtration efficiency at the lowest possible pressure drop. In this regard, the filtration efficiencies of many filters can be improved, without a corresponding increase in pressure drop, by electrostatically charging the materials in order to impart a charge to the filter media. The use of electrets for filtration applications has been known for some time. The advantage of materials of this type is that the charge on the fibers considerably augments the filtration efficiency without making any contribution to the airflow resistance. Air filtration efficiency varies with the electrostatic charge; however, it is not a direct measure of the quantity or magnitude of charge in the media,
It is known that certain dielectric materials can be permanently electrostatically polarized by various means including, for example, under the influence of the electric field. A dielectric becomes an electret when the rate of decay of the field-induced polarization can be slowed down so much that a significant fraction of the polarization is preserved long after the polarizing field has been removed. Such electrets can be made by various methods, e.g. corona charging, triboelectric charging (friction) and so forth. Methods of treating various materials to impart an electrostatic charge are described in U.S. Pat. No. 4,215,682 to Kubic et al., U.S. Pat. No. 4,375,718 to Wadsworth et al., U.S. Pat. No. 4,588,537 to Klaase et al. and U.S. Pat. No. 5,401,446 to Tsai et al. However, the ability to impart an electrostatic charge or field of sufficient initial strength and/or maintaining a desired level of electrostatic charge over time has proven difficult for many materials and, in particular, non-polar materials such as polyolefin fabrics. Moreover, many thermoplastic polymer materials often experience a significant or accelerated degradation in the level of electrostatic charge upon exposure to heat and/or moisture. In this regard, it will be readily appreciated that many filtration materials are exposed to heat and/or moisture such as, for example, HVAC filtration media, sterilization wraps, vacuum bag liners, face masks and so forth.
Various topical treatments have been used as a means to impart and/or improve the stability of electrostatic charges. Additionally, charge stability of nonwoven webs of non-polar polymeric materials has been improved by introducing polar groups onto side-chains and/or the backbone of the non-polar monomer or otherwise grafting unsaturated carboxylic acids thereon such as, for example, as described in U.S. Pat. No. 5,409,766 to Yuasa et al. Further, in an attempt to achieve a stable high charge density others have utilized polymeric materials comprising both polar and non-polar polymers. As an example, U.S. Pat. No. 4,626,263 to Inoue et al. discloses an electret treated film comprising a non-polar polymer and a non-polar polymer modified by grafting or copolymerization with a carboxylic acid, epoxy monomer or silane monomer. However, the use of copolymers or grafted polymers containing polar groups within or otherwise branched from the backbone of the host polymer can result in a polymer that is incompatible or immiscible with the host polymer. Immiscibility results in the formation of discrete domains of the copolymer and/or backbone grafted polymer within the host polymer. The host polymer thus forms a continuous phase and the copolymer and/or backbone grafted polymer being a discontinuous phase. The existence of discrete domains within the host polymer can result in a material having reduced tenacity, tensile modulus and/or increased opacity. Therefore, there exists a need for polymeric material having good electret stability with improved strength. Further, there exists a need for such highly charged materials that are capable of substantially maintaining its initial charge over time.
Unlike polymers having functional moieties added by copolymerization or backbone grafting, the term xe2x80x9ctelomerxe2x80x9d or xe2x80x9ctelechelicxe2x80x9d polymer refers generally to polymers having a reactive or functional end group. Telomers are known in the art and methods of making the same are described in U.S. Pat. Nos. 4,342,849 and 5,405,913 and Japanese 08/067704A2. Such polymers have traditionally referred to those polymers that contain a functional end group which can selectively react with or bond with another molecule. Telomers or telechelic polymers have heretofore been used as additives in various systems to impart additional adhesive or cross-linking properties to the same. In this regard, telomers have been used as a cross-linkable coating by incorporating reactive end groups. As an example, polyamide telomers having aryloyl end groups undergo cross-linking upon exposure to electron-beam radiation. Also, telomers have been added to adhesives systems in order to improve their function. For example, polyurethane polymer adhesives for bonding of metals exhibit higher peel strengths upon addition of phosphorous containing telomers. Telomers have also been used as processing aids for plasticizers and other materials. For example, processability of ethylene-propylene rubbers is said to be improved when various telomeric materials are added. Telomers have also been used as surfactants, biocidal agents, lubricants and other uses, examples of which are described in the Encyclopedia of Polymer Science and Engineering, vol. 16, pg. 549-551 (1989).
The problems experienced by those skilled in the art are overcome by the present invention which comprises an electret material comprising a blend of a first thermoplastic polymer and a substantially compatible telomer. In a further aspect of the present invention, an electret material is provided comprising a porous substrate such as a nonwoven web of thermoplastic polymer fibers having a permanent or stabilized charge contained therein and wherein at least a portion of the fibers comprise a blend of a first thermoplastic polymer and a telomer compatible with the first thermoplastic polymer. The telomer desirably comprises from about 0.1 to about 25% by weight of the polymeric portion of the film or fiber and even more desirably from about 0.5% to about 15% of the polymeric portion of the film or fiber. In a further aspect, the telomer desirably comprises a backbone substantially similar to that of the first thermoplastic polymer component. As an example, the porous substrate can be a nonwoven web of fibers which comprise from about 90% to about 99%, by weight, polypropylene and from about 1%-10%, by weight, polypropylene backbone with one or more functional end groups.